pgsql: Further fixes for quoted-list GUC values in pg_dump and ruleutil

Further fixes for quoted-list GUC values in pg_dump and ruleutils.c.

Commits 742869946 et al turn out to be a couple bricks shy of a load.
We were dumping the stored values of GUC_LIST_QUOTE variables as they
appear in proconfig or setconfig catalog columns. However, although that
quoting rule looks a lot like SQL-identifier double quotes, there are two
critical differences: empty strings ("") are legal, and depending on which
variable you're considering, values longer than NAMEDATALEN might be valid
too. So the current technique fails altogether on empty-string list
entries (as reported by Steven Winfield in bug #15248) and it also risks
truncating file pathnames during dump/reload of GUC values that are lists
of pathnames.

To fix, split the stored value without any downcasing or truncation,
and then emit each element as a SQL string literal.

This is a tad annoying, because we now have three copies of the
comma-separated-string splitting logic in varlena.c as well as a fourth
one in dumputils.c. (Not to mention the randomly-different-from-those
splitting logic in libpq...) I looked at unifying these, but it would
be rather a mess unless we're willing to tweak the API definitions of
SplitIdentifierString, SplitDirectoriesString, or both. That might be
worth doing in future; but it seems pretty unsafe for a back-patched
bug fix, so for now accept the duplication.

Back-patch to all supported branches, as the previous fix was.

Discussion: https://postgr.es/m/7585.1529435872@sss.pgh.pa.us

Branch
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REL9_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/12f2d814ac61a63c5dd1745fe0f0ff5f5202cd44

Modified Files
--------------
src/backend/utils/adt/ruleutils.c | 37 ++++++++++--
src/backend/utils/adt/varlena.c | 112 ++++++++++++++++++++++++++++++++++++
src/bin/pg_dump/dumputils.c | 109 +++++++++++++++++++++++++++++++++++
src/bin/pg_dump/dumputils.h | 3 +
src/bin/pg_dump/pg_dump.c | 30 ++++++++--
src/bin/pg_dump/pg_dumpall.c | 33 +++++++++--
src/include/utils/builtins.h | 2 +
src/test/regress/expected/rules.out | 26 ++++-----
src/test/regress/sql/rules.sql | 2 +-
9 files changed, 324 insertions(+), 30 deletions(-)